a) Field of the Invention
This invention relates to a real image mode finder used to zoom lens shutter cameras.
b) Description of the Prior Art
In finders for zoom lens shutter cameras with high variable power, Albada type and bright frame type finders are unsuitable for compact designs because their outer diameters of lenses become inevitably large, so that the use of a real image mode finder is effective.
In general, a Porro prism is used as a means for erecting an image in the real image mode finder. The Porro prism, however, needs to provide, upward or downward, space at least twice the height of an imaging plane involved in a finder objective lens and laterally space at least twice the width of the imaging plane. Further, in consideration of the layout relative to the inside of a camera body, a prism section is necessarily arranged just before an eyepiece lens, namely, in the vicinity of a film box. Consequently, if it is intended that the height of the body is reduced, the width of the body will be increased for the purpose of preventing interference with the film box, while if the width of the body is to be narrowed, the height of the body will be increased. As such, where the compact design of the body is taken into consideration, image erection involved in the Porro prism has come to a principal cause restricting the compact design.
Accordingly, as a space-saving prism configuration for the image erection, a Pechan prism is available such as is set forth in Japanese Patent Preliminary Publication No. Sho 59-229536, for instance.
This example, as shown in FIG. 1, is adapted to combine a triangular prism X with a triangular Dach prism Y having a Dach surface and is constructed so that light coming from an objective lens not shown, after reflection at a lower surface 1 of the triangular prism X, is reflected from an upper surface 2, emanates from the lower surface 1 to be incident on the triangular Dach prism Y, is totally reflected from a reflection surface 4 before being reflected twice by a Dach surface 3, further reflected from an incidence surface 5 to emerge from the emergence surface 4, and directed toward an eyepiece lens not shown.
The construction in which the Pechan prism is employed is such that each of the lower surface 1 of the triangular prism X and the incidence surface 5 and emergence surface 4 of the triangular Dach prism Y has the functions of reflection and transmission of light. If, however, it is intended that the light is transmitted perpendicularly through and totally reflected from each of these surfaces, the prism configuration will be useful only in the case where reflected light is made incident at an angle of 45.degree. on each surface. Consequently, in order to fulfil simultaneously the total reflection and transmission with respect to such each surface, it is necessary for the prisms to use materials with high refractive indices or to partially have reflection coatings. Moreover, in this type of configuration, two prism elements are required and the number of reflections is as large as six, so that the degree of angular accuracy of each surface must be increased. As such, the use of the Pechan prism as the image erection means leads to high cost and in particular, the use of the prism for products aimed at low cost, such as lens shutter cameras, has been very disadvantageous.